A majority of drug delivery stents and scaffolds include three components, (1) a scaffold/stent structure, substrate, or body, (2) a polymeric coating, and (3) a therapeutic agent typically present to reduce neointimal hyperplasia and restenosis. The primary purpose of the polymeric coating is to hold the drug on the scaffold and control its release rate. A bioresorbable scaffold includes a bioresorbable scaffold structure and a bioresorbable coating. Bioresorbable coatings are also placed on metallic stents with the goal of offering benefits compared to a permanent polymer coating.
As a drug/device combination product, drug delivery scaffolds must meet existing standards for drug release rate control and preservation of drug purity. An example of such a standard is the Q3B(R2) Impurities in New Drug Products standard, FDA July 2006. When a drug is mixed with a polymer it is possible for it to become chemically altered from reaction with the polymer itself, reaction with environmental species, or post packaging sterilization (such as terminal sterilization), among other mechanisms. Reaction of the drug with the carrier polymer, resulting in a drug that is conjugated to the coating polymer, is problematic as it both decreases the drug dose and generates new species with unknown biological and toxicological properties. Significant amounts of these species, where a significant level may be as low as 1%, necessitate complex and expensive studies to identify the new species and can require toxicological studies. Ultimately, substantial amounts of drug-polymer species can lead to studies resembling the regulatory path for a new drug substance, such as the US Food and Drug Administration (FDA) Investigational New Drug process (IND). This outcome can be prohibitively expensive and of little value, as drug degradation products and drug-polymer adducts rarely have any therapeutic value above that of the unaltered drug.